The new note was worth 1.25 U.S. dollars at Monday's black market exchange rate. A week ago, $50 billion was worth $2.20. Two weeks ago it was worth 3.30 U.S. dollars.

The Reserve Bank of Zimbabwe unveiled new $20 billion and $50 billion notes in an advertisement in Monday's Herald newspaper. It said the notes were being introduced "for your convenience." In December, the bank printed new $10 billion notes.

But each time a larger denomination note comes, prices shoot up in the shops even though local currency is so scarce that most demand that customers pay in U.S. dollars or South African rand.

Zimbabweans are reeling from the inflation. The largest note ever in the country was $100 billion in August, when the before the bank struck 10 zeros from the currency. Then The Herald newspaper cost 10 new Zimbabwe dollars. Today the paper costs $15 billion.

Zimbabwe's economy is in meltdown amid a months-long political crisis over President Robert Mugabe sharing power with the opposition amid acute shortages of everything from medication and food to fuel and electricity. A collapse of the piped water system has caused a cholera epidemic that has killed nearly 2,000 people.